202 Mr Brooke's Remarks on Dr Brewster's Reply 
But the circumstance most likely to embarrass the mineralo- 
gist, even if his road through Dr Brewster's system were clear 
in every other respect, is the utter confusion of forms which one 
branch of Dr Brewster's theory would establish among crystals. 
44 If," says Dr Brewster, 44 we take a crystal bounded by six 
equal square faces , the crystallograplier will content himself 
with calling it a cube ; but the optical mineralogist will only 
call it it a cube when it has no double refraction ; he will main- 
tain it to be a rhomb ( qu . rhomboid), if it has a single axis of 
double refraction , coincident with one of its diagonals ; and he 
will consider it as a right prism with a square base , if it has a 
single axis of double refraction , perpendicular to any two of its 
faces? 
To this Dr Brewster will allow me to add, and he will main- 
tain , that it belongs to the prismatic system of Mohs , if it has 
two axes of double refraction . 
Thus, each of the classes of the primary forms might , in its 
turn , be personated by the cube ; — a metamorphosis not very 
consistent with the notion of form, upon which crystallographi- 
cal distinction has been hitherto founded. Let this difficulty, 
however, be overcome, and let the mineralogist be contented to 
talk of a rhombic or doubly oblique prism , contained within six 
square planes , still he will find, that in resorting to his pola- 
rising mirrors , he has not substituted for his goniometer an in- 
strument capable of affording a greatly increased facility to his 
researches. For Dr Brewster has shewn that his optical method 
presents obstructions to the mineralogist, which would sometimes 
demand extraordinary perseverance and good fortune for the 
detection of a single species. He says, in p. 218. of the Phil. 
Trans, for 1818, 44 The extreme difficulty which attends experi- 
ments of this kind will be understood from the fact, that I cut 
more than ffteen plates out of a large piece of zircon without 
discovering its axis. By a singular accident , however, Mr 
Morton, a jeweller in Edinburgh, procured for me no less than 
sixty plates of zircon with parallel faces , and it was only in two 
of these that the system of rings was developed." But what is 
the mineralogist to do who is not in the way of such another 
singular accident P 
Under all these circumstances, I may appeal again to the 
